The present invention generally relates to termination impedances and more particularly to dynamically controlling termination impedances for input and output pins.
Data rates at integrated circuit input and output pins have increased dramatically the last several years, and further increases are sure to follow. These high data rates place strict requirements on signal integrity. Transmitted and received signals need to be substantially free from glitches and other undesirable artifacts to maintain the highest possible data rates.
Reflections due to impedance mismatches are one source of these artifacts. Reflections are caused when a signal is received from a transmission line by a receiver having an impedance that does not match the impedance of the transmission line. In short, this mismatch prevents some of the energy of the signal from being absorbed by the receiver. This unabsorbed energy is reflected back to the signal source. To prevent these reflections, a receiver termination impedance is connected from a receiver input to an AC ground, where the termination impedance matches the transmission line. Similarly, reflections can be caused by mismatches between a transmitter and its transmission line, so a source termination impedance is connected between a transmitter output and the transmission line.
The receiver termination impedance is in parallel with the receiver, so it is referred to as a parallel termination. Similarly, a source impedance is in series with a transmitting circuit, and is referred to as a series impedance. When signaling is limited to one direction, fixed series and parallel terminations can be used. A difficulty arises when bidirectional (or full duplex) signaling is used. A parallel termination used to receive signals creates an impedance mismatch when transmitting data. Similarly, a series termination used when transmitting causes an impedance mismatch when data is received.
It is desirable to be able to switch these impedances in and out of their transmitting and receiving circuits. That is, when one integrated circuit is transmitting, that integrated circuit should have a series termination, but when it is receiving, a parallel termination is required.
Accordingly, what is needed are circuits, methods, and apparatus that provide dynamic control of these series and parallel termination impedances.